Look closely at the image above, and take note of two things.
First, the blackboard on the wall isn't casting a shadow on the floor. This is something that's been irritating us for ages. Now, with the interior lighting setting switched on, objects mounted on walls don't cast shadows when the walls don't cast shadows.
Now compare the two pictures on the right. (This is a bit subtler; you may need to click on the image and see it a bit larger.) The left-hand picture casts a shadow on the wall, but the right-hand one doesn't. That's because they're placed in different ways. The left-hand one is actually fractionally in front of the wall, and was placed there with the gizmo. The right-hand one is attached to the wall normally, and so has no shadow.
This now means you have the ability to have shadows if they're important to you, or lose them when they're not working as you'd expect.